Field of the Invention
One disclosed aspect of the embodiments relates to a power supply apparatus and an image forming apparatus.
Description of the Related Art
Up to now, a power supply apparatus provided with a transformer, a driving circuit that drives the transformer, and a rectifier circuit and configured to generate a direct-current high voltage by rectifying and smoothing a voltage boosted by the transformer has been proposed as a power supply apparatus for an image forming apparatus that adopts an electrophotographic method.
In the image forming apparatus based on the electrophotographic method, first, an optical image corresponding to an image that is desired to be copied is projected onto an image bearing body charged to have the same polarity as a charging polarity of toner via a charging member such as a charging roller, and an electrostatic latent image pattern is obtained. Next, charged toner is supplied, and the latent image pattern is caused to absorb this toner to be developed. A recording sheet is overlapped with the developed toner to be applied with a polarity opposite to the charging polarity of the toner by a transfer member, and charges are uniformly applied to a back surface of a transferred medium such as paper, so that a toner image formed on the image bearing body is electrostatically transferred onto the transferred medium. Thereafter, a fixing device applies heat and pressure to the transfer paper to fix a transferred developing agent image, and image formation onto the recording sheet is ended.
In the above-described transfer process, the charges need to be appropriately applied to the back surface of the transferred medium. However, a resistance value of a transfer roller functioning as the transfer member which uses a general ionic conductive material largely changes depending on a moisture content contained in the air, and the transfer roller has a feature of being easily affected by an environment (temperature, moisture). For this reason, to appropriately supply the charges to load having a changing resistance value, a high voltage power supply apparatus that applies bias to the transfer roller generally adopts a constant current control method.
When the toner is adhered onto the above-described transfer roller surface, a cleaning operation for moving the toner adhered onto the surface to the image bearing body is performed by applying a voltage having the same polarity as the charging polarity of the toner. At this time, a voltage used for the cleaning operation needs to be higher than a charging voltage of the image bearing body (the voltage needs to be lower in the case of a negative polarity).
To realize the above-described electrophotographic process, an image forming apparatus in related art is provided with a transformer configured to generate necessary bias for each of biases such as a charging voltage, a development voltage, and a transfer voltage. However, in recent years, miniaturization and cost reduction in the image forming apparatus have been demanded by the market, and an attempt to generate a plurality of biases by a common transformer has been proposed as described in Japanese Patent Laid-Open No. 2007-206414.
In a case where the plurality of biases are output by the common transformer in the power supply apparatus as described above, advantages are attained that the number of the transformers can be decreased, and the apparatus can be miniaturized. On the other hand, an approach for miniaturizing the transformer itself is also important. Here, in a case where the miniaturization of the transformer is considered, for example, a method of reducing a transformer output is exemplified. However, even when bias outputs such as charging, development, and transfer in image formation are to be reduced, the bias outputs are not easily changed in view of maintenance of an image quality.